A1 alloy and method

ABSTRACT

An aluminium alloy in the AA5XXX series has the composition: Si 0.10-0.25 %; Fe 0.18-0.30 %; Cu up to 0.5 %; Mn 0.4-0.7 %; Mg 3.0-3.5%; Cr up to 0.2%; and Ti up to 0.1%. Rolled and annealed sheet of the alloy is readily formed into shaped components for use in vehicles which components have good strength and resistance to stress corrosion cracking.

This invention is concerned with a new alloy in the 5000 Series of theAluminum Association Register. Ingots of the alloy can be converted torolled sheet which can be formed into shaped components for use invehicles.

Non-heat-treatable alloys of the Al—Mg (5xxx) type are well suited tothe application of automotive structural pressings to form abody-in-white structure. In the soft annealed condition (O-temper) thesealloys can have high formability allowing the complex structurepressings to be manufactured. Subsequent heat treatment during the carmanufacture (e.g. paint-bake ovens) reduces the as deformed strengthback close to the O-Temper properties due to thermal recovery. Unlikeheat-treatable alloys, these properties are then stable throughout thelife of the vehicle, i.e. no artificial ageing takes place.

The alloy AA5754 is a well known non-heat-treatable 5xxx series alloy,(2.6 to 3.6% wt Mg). The specification, given in Table 1, is broad andas such far too wide for the automotive industry. The Mg level must becontrolled to tighter limits to maintain an acceptable spread of proofstress values in the final sheet. Also, to render the alloy sufficientlyformable, it is usually based upon low Si and Fe (about 0.08% wt Si andabout 0.2% wt Fe) requiring virgin smelter metal. Such alloys are notreadily recyclable because during each remelting the Si and Fe levelsincrease and quickly exceed the level at which formability declines.There is a need for an alloy that can be recycled. This is particularlytrue of alloys intended for use in the mass production of automobiles.Alloys which require smelter metal obviously are not recyclable.

TABLE 1 AA5754 Si Fe Cu Mn Mg Cr Zn Ti AA 5754 Max 0.40 0.40 0.10 0.503.6 0.30 0.20 0.15 Limits Min 2.6

Conventional 5xxx series structural alloys have either lower strength,due to a reduced Mg and Mn level (such as AA5251 and AA5754), or haveequivalent/greater strength but are sensitive to intergranular corrosionand Stress Corrosion Cracking (such as AA5182).

This invention relates to the development of an alloy composition andprocessing route which gives rise to a higher strength 5xxx series alloywhich is insensitive to SCC, and tolerant to high levels of Si and Fe interms of formability. A characteristic of the current alloy is thatbecause it can contain high levels of Si and Fe, it is therefore morerecyclable.

In one aspect the present invention provides an alloy of composition inwt %:

Si 0.10-0.25 preferably 0.10-0.20

Fe 0.18-0.30 preferably 0.20-0.30

Cu up to 0.5 preferably up to 0.3

Mn 0.4-0.7 preferably 0.4-0.5

Mg 3.0-3.5

Cr up to 0.2 preferably up to 0.1

Ti up to 0.1

Others up to 0.05 each, 0.15 total

Al balance

This is a relatively high-strength alloy, it has a 0.2% proof strengthof 105-110 MPa, compared to 90-95 MPa for the standard AA5754 alloycontaining 2.9 wt % Mg.

Components for load bearing structures in automobiles are press formedwhich involves stretch forming and deep drawing Deep drawing is oftenthe most important process, and this calls for a high r value, that isto say a high plastic strain ratio, that is uniform in the plane of thesheet. This need is met by the alloys of the invention.

Mg is the principal solid solution strengthening addition in the alloy.The Mg content of the alloys of this invention, which is relatively highat 3.0-3.5%, results in increased strength and formability. However, ifthe Mg level is raised too far, then intergranular corrosion and stresscorrosion cracking (SCC) problems, associated with the formation of anAl₈Mg₅ precipitate at grain boundaries, restrict performance. For batchannealed material, an upper limit of Mg is set at 3.3%. For continuouslyannealed and solution heat-treated (CASH) material, the Mg content maybe pushed up as high as 3.5%.

Mn is present at relatively high levels of 0.4-0.7% preferably up to0.6% more preferably up to 0.5%. Homogenisation of the alloy results inprecipitation of α-AlMnSiFe particles which give rise to additionaldispersoid strengthening. Very high Mn levels are detrimental due to theformation of a coarse intermetallic phase MnAl₆. The increased densityof dispersoids causes a refinement of the O temper grain size and aresultant increase in strength.

Cu may be present at levels up to 0.5% preferably up to 0.3%, morepreferably up to 0.10%. At higher levels (e.g. up to 0.3%), Cu givesrise to significant strength retention after a paint bake cycle. Above0.3% no additional benefit is obtained. Cu is an inevitable impurity inrecycled scrap. Cu levels above 0.15% give rise to alloys having high rvalues but which may (unless the working conditions are rather closelycontrolled) be detrimental by virtue of very pronounced variation in theplane of the sheet (high Δr).

Si is present at 0.10-0.25% preferably up to 0.20% and improvesstrength. High Si and Mn have surprisingly been found to improve the rvalue of sheet and to promote uniformity in the plane of the sheet (lowΔr). But Si content as high as 0.3% gives rise to reduced ductility andreduced formability.

Fe is specified at 0.18-0.30% preferably 0.20-0.30%. Fe contributes todispersion strengthening, but at high concentrations lowers formability.

The Si and Fe levels are set such that the alloy can be produced fromrecycled metal. Recycling increases the Si and the Fe levels in thecharge. It also increases the Cu content. The new alloy of the inventionis more tolerant of these impurities.

Cr has similar effects to Mn and may be used in partial replacement ofMn. Preferably the (Cr+Mn) content is at least 0.4%. Preferably Cr isnot deliberately added to the alloy, i.e. is present only as anincidental impurity at up to 0.05%.

Ti may be added to refine the grain structure.

Other alloying components may be present in minor concentrations up to0.05% each, 0.15% total. Components deliberately added may include Znand B. Other components would normally be present only as adventitiousimpurities. The balance of the alloy is Al.

In another aspect the invention provides rolled and annealed sheet ofthe alloy described. (Rolled sheet for canstock is used in a hardas-rolled condition). The following paragraphs describe the processingsteps used to produce that rolled sheet.

Molten metal of the required composition is cast, typically by directchill casting although the casting technique is not material to theinvention. An ingot of the alloy is homogenised, preferably at arelatively high temperature of at least 500° C. preferably 530-5800° C.particularly 550-580° C., for 1-24 hours. Homogenisation is preferablyperformed under conditions that result in the formation of a finedispersoid of α-AlMnSiFe particles. If the homogenisation temperature istoo low, it is possible that this may be produced as a coarserneedle-like precipitate which exhibits growth with increasedhomogenisation time. These needles can break up during rolling to createvoiding in the structure, resulting in possible reduced ductility.Homogenisation at sufficiently high temperature results in sphericalprecipitates being formed which do not break up during rolling. Thesedispersoids are also relatively stable in size with homogenisation timesup to 16 hours and possibly beyond.

The homogenised ingot is then hot rolled and cold rolled, both underconditions which may be conventional. During cold rolling, aninteranneal is optional, preferably at a temperature of 300-400° C. inbatch operation or at 400-550° C. in continuous operation. When aninteranneal is employed, a final cold rolling treatment results in athickness reduction preferably in the range 40-60% e.g. about 50%. Afinal annealing step, preferably at 300-400° C. for 0.05-5 hours inbatch operation, or at 400-550° C. in continuous operation, may becarried out on a batch basis, or as a continuous anneal and solutionheat treatment. Annealing conditions should be such as result in a fullyrecrystallised grain structure i.e. one produced by high angle grainboundaries sweeping through the structure. Such alloys have goodformability and high elongation to break.

The resulting rolled sheet has the aforementioned combination of desiredproperties: high strength, insensitive to stress corrosion cracking andtolerant to high levels of Si and Fe in terms of formability. The sheetwill be useful for forming into components to be joined together, e.g.by adhesive bonding or weld bonding or mechanical fastening to formstructures e.g. load-bearing structures of motor vehicles.

The alloys used in Example 1 are set out in Table 2 below. Of these, STDis a typical M5754 standard composition; 1, 2, 3 and 4 are in accordancewith the present invention.

TABLE 2 ALLOY Si Fe Cu Mn Mg Cr Zn Ti STD 0.068 0.21 0.001 0.26 2.92 — —0.012 1 0.16 0.25 0.002 0.44 3.24 — — 0.013 2 0.16 0.25 0.15 0.43 3.36 —— 0.012 3 0.22 0.24 0.002 0.43 3.25 — — 0.012 4 0.21 0.24 0.151 0.433.28 — — 0.012

Reference is directed to the accompanying drawings in which:

FIG. 1 sets out the casting and processing schedule of the alloysdescribed in Table 1.

Each of FIGS. 2 to 14 is a bar chart comparing a particular featurebetween different alloys or different processing routes.

Alloys having compositions set out in Table 2 were DC cast and processedin the Laboratory to 1.6 mm gauge sheet according to the schedule set inFIG. 1. Inter-anneals and the final batch anneal were carried out at330° C. for 2hrs. followed by air cool. The sheets were subjected to thefollowing tests:

i) Tensile test parameters as a function of orientation

ii) Erichsen value

iii) Hydraulic bulge height and thickness failure strain (logarithmic)in balanced bi-axial tension

iv) Plane strain tension limit strains

v) r value as a function of orientation

vi) R/t bend test. (R=inner bend radius, t=material gauge)

Standard ASTM E8 tensile specimens were used to generate the standardtensile data of proof, UTS, uniform and total elongation, in the threemajor directions. From the data, strain hardening index values (n) werederived.

Erichsen values were obtained using the standard test procedure andgeometry, with a polyethylene film used as a lubricant between thetooling and the sheet material.

The bulge height and thickness failure strains were determined using ahydraulic bulge testing machine that rigidly clamps a sheet of materialusing a draw bead section machined on a 175 mm pitch circle. Sheetthickness was determined after bulging of the material using anultrasonic probe, from which the failure strain was determined.

Plane strain tension limit strains were determined by using a fixturethat offered transverse restraint to the tensile specimens via the useof knife edges. (Technique reference: Sang H., Nishikawa Y., A PlaneStrain Tensile Apparatus. J. Metals, 35(2), 1983, pp30-33).

The r values were determined using JIS#5 tensile specimens, (50 mm gaugelength, 25 mm width), the increased width giving rise to more accuratewidth strains and hence r values.

R/t bend tests were carried out by bending the material according toASTM Designation E 290 92. This apparatus was used to bend samplesthrough approximately 150°, after which they were squeezed to a 180°bend in a vice. The outer surface of the bend was then examined forevidence of orange peel/cracking for the different radii used in thetrials.

Additionally, standard ASTM E8 tensile specimens were pulled to both 2%and 5% strain, and then subjected to a standard paint bake cycle of 180°C. for 30 minutes to assess whether Cu additions up to 0.15% wt wouldgive rise to any significant strength retention after a paint bakecycle.

Stress Corrosion Cracking, (SCC), sensitivity was assessed via slowstrain rate testing, (1×10⁻⁷ per second). Specimens were pre-strained20% followed by sensitisation at 150° C. for varying times, and thentested under both dry conditions and immersed in a salt/peroxidesolution (3% NaCl 0.3% H₂O₂). The elongation to failure for each testwas recorded, and plotted for individual conditions as a ratio of thewet to dry performance. A ratio of one indicates no sensitivity to SCC.

Homogenisation at 540° C. produced needle like precipitates in the castingot, whereas the higher temperature treatment at 560° C. resulted inthe formation of a spherical precipitate. This spherical precipitate wasvery resistant to coarsening over homogenisation times up to 16 hours attemperature.

After cold rolling, the grain size of the high Cu high Si alloy 4 wasfiner than in the standard alloy, and the higher reduction resulted in afiner grain size. The low temperature homogenisation gave a finer grainsize, (FIG. 2).

Proof and tensile strength of the alloys are compared in FIGS. 3 and 4.Comparing alloy 1 with the standard alloy STD reveals the strengtheningeffect of the higher levels of Mg and Mn. Also, this has been achievedwith minimum reduction in formability in spite of the increased levelsof Si and Fe, FIGS. 5 and 6

The Erichsen test data are shown in FIG. 7.

The hydraulic bulge height data, and the bulge thickness failure straindata, are shown in FIGS. 8 and 9 respectively. The properties of 1 aredistinctly superior to those of 2, 3 and 4.

FIGS. 10 and 11 compare the r values of the sheets. 1 and 3 have thebest combinations of high r value and little variation in the plane ofthe sheet (Δr). The Cu containing alloys had higher average r values butvery pronounced variations (Δr) in the plane of the sheet.

FIGS. 12 and 13 show respectively longitudinal R/t bend test data andtransverse R/t bend test data.

EXAMPLE 2 Stress Corrosion Cracking Batch and Continuously AnnealedSheet

Stress corrosion cracking was measured on experimental alloys rolled andprocessed on a commercial mill. Stress corrosion cracking is caused bythe precipitation of a continuous film of Al₈Mg₅ on grain boundaries andthis process is substantially independent of the Si or the Mn contentsof the alloy. The amount of these elements in the test alloys istherefore substantially irrelevant to the results obtained. Theimportant element is Mg.

The composition and the process schedule for the alloys 5 and 6 are setout below:

Alloy

5. 3.49% Mg, 0.59% Mn, 0.06% Si, 0.22% Fe

6. 3.44% Mg, 0.63% Mn, 0.15% Si, 0.19% Fe

Processing Route

Batch annealing was compared with continuous anneal of alloy 5 rolled ona commercial mill according to the following schedule:

DC cast 600 mm ingot

Homogenise 550° C. for 9 hours

Hot roll to 4.2 mm (self anneal reroll)

Cold roll to 1.6 mm final gauge

Either

(1) Batch anneal (BA), heating at 50° C./hr to 330° C. and soak for 2hours

or

(2) Continuously anneal (CAL) at 450° C. peak metal temp and forced aircool.

An evaluation of the sensitivity to SCC was made. The comparison metalswere: a commercial AA5182 alloy containing 4.5% Mg, a commercial AA5754batch annealed alloy having a composition close to STD and alloy 1 fromExample 1.

Resistance to stress corrosion cracking of these alloys after a batchanneal is shown in FIG. 14. The batch annealed 3.25% Mg alloy has goodstress corrosion resistance whereas the similarly treated alloys 5 and 6containing 3.49% and 3.44% Mg show a marked reduction in stresscorrosion cracking resistance. However the continuously annealed alloy 5showed improved stress corrosion cracking resistance, and the same wouldhave been the case, it is believed, for a continuously annealed alloy 6.

EXAMPLE 3 Another Alloy

Al alloy 7 had the composition in wt %:

Mg 3.41 Mn 0.45 Fe 0.244 Si 0.14

Processing Route

Ingot preheat −540° C.

Hot rolled to 3.5 mm (re-roll gauge).

Cold rolled to 1.6 mm (final gauge).

Cold reduction 54%.

Final anneal −340° C.

Properties 0.2% Yield Stress (MPa) Longitudinal 114 45° 109 Transverse113 Total Elongation (%) Longitudinal 20.1 45° 24.5 Transverse 24.1Formability (depth/height, mm) 10 cm draw 32 20 cm plane strain 26Biaxial 42 r/t Bend Longitudinal 0.12 Transverse 0.06 Erichsen domeheight (mm) 9.6

For an alloy that can be made from recycled metal, rather than smeltermetal, these properties are satisfactory.

What is claimed is:
 1. An alloy of composition consisting essentially of(in wt %): Si 0.10-0.25 Fe 0.18-0.30 Cu up to 0.15 Mn 0.4-0.5 Mg 3.0-3.5Cr up to 0.2 Ti up to 0.1 Zn  up to 0.05 B  up to 0.05 Unavoidableimpurities up to 0.05 each, 0.15 total Al balance,

wherein the alloy comprises recycled metal, and wherein sphericaldispersoids of α-AlMnSiFe particles are present.
 2. Rolled and annealedsheet of the alloy of claim
 1. 3. Automobile structural components madefrom the sheet of claim
 2. 4. A sheet as claimed in claim 2 which isinsensitive to stress corrosion cracking.
 5. An alloy as claimed inclaim 1, comprising 0.10-0.20 wt. % Si.
 6. An alloy as claimed in claim1, comprising 0.20-0.30 wt. % Fe.
 7. An alloy as claimed in claim 1,comprising up to 0.1 wt. % Cr.
 8. An alloy as claimed in claim 1,wherein Si is present in an amount of 0.16-0.25 wt %.
 9. An alloy asclaimed in claim 1, wherein Si is present in an amount of 0.14-0.25 wt%.
 10. A method of making the sheet of claim 2 comprising the steps:casting; homogenising at 550-580° C.; hot rolling; cold rolling;optional interannealing; final cold rolling; final annealing wherein thehomogenisation is performed under conditions that result in theformation of spherical dispersoids of α-AlMnSiFe particles.
 11. A methodas claimed in claim 10, wherein: casting is by DC casting; optionalinterannealing is at 300-400° C. in batch operation or at 400-500° C. incontinuous operation; final cold rolling is to a 40-60% reduction; finalannealing is at 300-400° C. in batch operation or at 400-550° C. incontinuous operation.
 12. A method as claimed in claim 11, wherein thealloy contains 3.0-3.3% Mg and final annealing is performed on a batchbasis.
 13. A method as claimed in claim 11, wherein the alloy contains3.2-3.5% Mg and final annealing is performed continuously.
 14. A methodas claimed in claim 10, wherein homogenizing is at 560-580° C.